1. Field of the Invention
The present invention relates to a press apparatus for removing water from a web. More particularly, the present invention relates to a press apparatus for removing water from a web, the press apparatus including a press shoe and a blanket enclosing the press shoe.
2. Background Information
An extended nip or shoe press was born in the 1980's for liner and corrugating medium papermaking machines with the aim of achieving the highest ex press dryness values. To reach the objective of achieving these high dryness values, high nip loads (from 1050 to 1500 kN/m) and long nips (standard 250 mm) were the main features of those shoe presses.
During the 1990's shoe presses were developed for application to fine paper grades due to the evidence that for the same dryness achievable with a roll press, a shoe press was able to give higher paper bulk or for the same bulk a higher ex press dryness. For fine paper applications it was discovered that in order to achieve the common values of bulk, the specific pressure under the nip had to be reduced to 50 60% of the pressure normally used on fluting and liner applications. Accordingly, this resulted in typical nip loads within a range 450/550 kN/m.
Medium or high nip loads of necessity mean that the shoe press and counter roll construction must be quite heavy and relatively large in size. Consequently, due to the relatively massive size and weight of typical extended nip presses, the impact especially in rebuild projects was in certain respects negative. More specifically, the advent of the shoe press had quite a big impact on the press layout (due to the relatively large size), on the press frame, machine building, lifting crane capacity (due to the high nip forces, and heavy weights involved) and finally on the time required from the request for such rebuild until operation of such rebuild. For the aforementioned reasons several shoe press projects have been cancelled because the projected total cost was out of budget. Additionally, because of uncertainty about return on investment and the rebuilding shut down time being too long, such rebuilds were often considered not acceptable for production continuity.
Accordingly, the basic concept of the invention is to provide a small and light shoe press unit (700 mm dia.), called a mini shoe press providing an extended nip with reduced nip load, (max 200 kN/m) and a proportionally reduced nip length (70 100 mm only). The lighter load combined with shorter nip according to the present invention provides the same specific pressure under the nip as a typical conventional shoe press unit.
The results in terms of bulk/dryness gains according to the present invention are somewhere midway between a conventional roll press and a standard shoe press. This has to be evaluated together with the lower impact in terms of budget, press layout, framing and building, shut down time associated with the mini shoe press installation according to the present invention compared with a conventional prior art shoe press.
Accordingly, the return on investment (ROI) is favourable to the mini shoe press arrangement according to the present invention.
Because of the unique size of the mini extended nip press arrangement according to the present invention, certain features of the structure and the pressures used are unique to the arrangement according to the present invention.
More specifically, the mini shoe press head design incorporates the blanket mechanical clamping concept disclosed in co-pending application No. PCT/EP01/01960.
Accordingly, the radial compactness of the system according to the present invention enabled the reduction of the mini shoe press outside diameter to 700 mm. However, if a typical clamping system used on a more conventional enclosed shoe press, with the exception of the arrangement disclosed in the aforementioned pending application were to be used, the attainment of such a small outside diameter would have been very difficult.
Another important feature of the present invention is the provision of an arrangement which permits adequate removal of lubricant from the enclosure defined by the blanket.
In a conventional shoe press, lubrication oil is pumped (hundreds of litres per minute that is 1 pm) inside the unit or enclosure. After use, the lubricant or oil is then collected inside the unit or enclosure into saveall pans and then conveyed to the outside of the unit through a relatively large channel ( 170/190 mm diameter) drilled into the back side journal. The inside or enclosure of the unit is also pressurized by a relatively light air pressure (35 mbars). The objective of the light or slight air pressure is to provide the required machine direction blanket tension. To be able to stay pressurized, the inside of the shoe press is perfectly sealed from the outside.
The oil discharge channel is sealed by a syphon to prevent internal air from leaking through the oil free portion of the channel above the oil bath or pan.
According to these principles, oil flows out of the unit purely assisted by gravity. This system works quite well as it is well balanced. More specifically, the amount of oil that is pumped into the unit is the same as the amount of oil drained by gravity out of the unit. Such flow of lubricant through the unit or enclosure does not interfere with the air pressure inside the unit. The only disadvantage is that this system as applied to conventional enclosed shoe presses requires quite a big cross sectional area for the drain channel due to the relatively low speed of the oil flowing out just by gravity. This feature is not a problem on a relatively large size unit thanks to the relatively massive size of the journals.
However, in a mini shoe press according to the present invention, the amount of oil required for the lubrication of the shoe is of the same magnitude as for a conventional shoe press. On the other hand, due to the reduced size of the journals in the mini press according to the present invention, it would be almost impossible to have a reasonable size for the drain channel to remove the oil in a passive way by gravity. Therefore, the oil extraction must be assisted by a suction pump to increase the oil flow speed and thus permit a reduced channel cross sectional area size. On the other hand it is practically impossible to have both injection and extraction of oil operated by pumps. More particularly, it would be impossible to have a constantly balanced system. Also, the air pressure inside the unit would be unstable. For example, if the suction pump sucks an amount of oil B which differs even only slightly from the amount of oil A pumped by the supply pump, the result would be that either 1/ the oil little by little fills the unit (when A is bigger than B) or 2/ the suction pump from time to time sucks air to compensate for a lack of oil (when B is bigger than A) which would result in instability of the air pressure inside the unit or enclosure. On the other hand it is practically impossible to maintain constantly the ideal condition in which A=B.
According to the concept of the present invention, the aforementioned problems are overcome by the provision of a supply pump having a pumping capacity A which is slightly greater than the capacity of the suction pump B.
Inside the mini shoe press unit or enclosure, two oil pans are provided. One of the pans, that is a primary pan is connected through an overflow to a secondary pan.
All the amount of oil A will be primarily collected by the primary pan from which the suction pump will take the amount B. However, B<A. Corresponding to the full capacity of the suction pump, the quantity B will be evacuated out of the unit through a hole the cross sectional area or size of which can be very small due to the high flow speed given to the oil by the suction pump.
The difference A−B, which=C will be transferred to the secondary pan through the overflow.
The amount of oil C will passively flow out of the unit or enclosure by gravity through a syphon similar to the type of syphon used in a conventional shoe press. The amount of oil C being only a small fraction of A such as 10%, only a channel of relatively small cross sectional area will be needed through the back side journal of the mini shoe press unit. More specifically, the channel will have a diameter of 80-100 mm instead of 170-190 mm needed in a conventional enclosed shoe press.
In summary, according to the present invention, there will be two channels to remove the oil from the unit or enclosure. Both of the channels will be relatively small in cross sectional area and they will easily fit into the small journal of the mini shoe press. The primary drain channel will be connected to the suction pump. Most of the oil (B, for example, 90% of total) will flow through this channel at a high velocity due to the pumping action. The secondary channel will be assisted only by gravitational force. The second channel will drain the rest of the oil (C, for example, 10%) at low velocity.
Therefore, it is a primary feature of the present invention to provide a press apparatus for removing water from a web that overcomes the problems associated with the prior art arrangements.
Another feature of the present invention is the provision of a press apparatus for removing water from a web that simplifies the manufacture of a rebuild papermaking press section.
A further feature of the present invention is the provision of a press apparatus for removing water from a web that reduces the cost of a rebuild papermaking press section.
Other features and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description of a preferred embodiment of the present invention contained herein.